UNIVERSITY    OF    CALIFORNIA 

COLLEGE    OF    AGRICULTURE 
AGRICULTURAL   EXPERIMENT   STATION 

CIRCULAR  No.  267 
June,  1923 

THE  TENDENCY  OF  TRACTORS  TO  RISE  IN 
FRONT;  CAUSES  AND  REMEDIES 

By   A.    H.    HOFFMAN 


Since  the  use  of  lighter  tractors  has  become  more  general,  a  number 
of  serious  accidents  due  to  tractors  'rearing  up'  in  front  have  occurred. 
The  causes  of  tipping  up  are  found  in  the  reaction  forces  necessarily 
present  in  a  tractor  when  it  is  in  operation.  The  purpose  of  this 
circular  is  to  explain  the  action  of  these  forces  and  to  suggest  methods 
of  controlling  them. 

When  any  tractor  is  pulling  a  heavy  load,  the  teeth  of  the  drive 
pinion  push  with  very  great  force  against  the  teeth  of  the  large  gear 
directly  attached  to  the  drive  wheel  or  track  sprocket.  If  a  worm 
drive  is  used,  the  threads  of  the  worm  push  against  the  teeth  of  the 
worm  wheel.  The  shaft  of  the  drive  pinion  (or  worm)  is  held  to  the 
tractor  frame  by  its  bearings,  enabling  it  to  exert  its  driving  force 
and  still  maintain  its  position.  It  is  only  by  'bracing  itself,'  by  push- 
ing backward  against  its  bearings  and  the  tractor  frame,  that  the 
pinion  can  exert  its  forward  force.  In  figure  1  the  arrows,  1,  2,  and 
3  show  the  directions  of  the  motion  of  the  tractor,  drive  wheel,  and 
drive  pinion,  respectively.  Arrow  'a'  shows  the  direction  and  point 
of  application  of  the  driving  force.  Arrow  'b'  shows  the  direction 
of  the  reacting  force  with  which  the  pinion  pushes  backward  against 
the  frame.  In  a  rear-drive  tractor  this  force  'b'  tends  to  revolve 
the  tractor  frame  up  in  front  and  over  backward  about  the  rear 
axle  as  an  axis.  It  makes  no  difference  whether  the  pinion  (or  the 
worm)  is  placed  above  or  below,  to  front  or  to  rear,  this  tipping-itp 
tendency  is  present  just  the  same  in  any  track  drive  or  rear-wheel  drive 
tractor  when  pulling  forward.  In  a  front  drive  tractor  the  effect 
of  the  driving  force  is  to  press  down  on  the  rear  carriage,  or  on  the 
implement  if  it  is  directly  connected,  when  pulling  forward;  and 
to  tip  up  in  the  rear  when  in  reverse.  In  a  tractor  driving  equally 
on  all  four  wheels  there  is  little  tendency  to  rise  in  front,  since  the 
center  of  weight  is  farther  forward  and  half  the  force  is  applied  to 
the  front  wheels. 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Fig.  1. — The  tendency  of  a  tractor  to  rise  in  front  is  due  to  certain 
forces  which  are  necessarily  present  when  the  tractor  is  working.  Arrows  1, 
2,  and  3  show  directions  of  motion;  arrow  'a'  direction  and  point  of  appli- 
cation of  driving  force;  arrow  'b'  the  reacting  force  with  which  the  pinion 
pushes  against  the  tractor  frame,  thus  tending  to  make  it  revolve  about  the 
axle  of  the  drive  wheel. 


Fig.  2. — A  track  type  tractor  has  the  same  tendency  as  a  wheel  type  to 
rise  in  front;  however,  the  higher  it  rises  the  less  becomes  the  tendency  to 
rise.  Many  track  type  tractors  are  steered  by  the  tracks,  hence  may  be 
turned  even  when  the  front  wheel  is  up  off  the  ground. 


Circular  267]     TENDENCY   OF  TRACTORS  TO  RISE  IN  FRONT  3 

In  most  tractors  this  tendency  to  tip  up  is  not  great  enough 
to  throw  the  tractor  over  backward  unless:  (a)  it  is  going  up  a  very 
steep  grade,  (b)  the  drivers  have  been  made  to  dig  themselves 
in  by  revolving  when  stalled,  or  (c)  the  clutch  is  thrown  in 
suddenly  when  the  engine  is  racing.  Track-type  tractors  are  less 
likely  than  the  wheel-type  to  go  over  backward,  though  the  fronts  of 
the  track  types  start  to  rise  when  the  load  is  very  moderate  or  even 
when  there  is  no  load  at  all  (fig.  2).  This  stability  of  the  track  type  is 
due  to  the  fact  that  the  middle  of  the  tread  area  still  touching  the 
ground  moves  backward  as  the  tractor  tips  up  in  front.     This  means 


Fig.  3.— Fin  flange  for  Fordson  tractors.  Attached  by  clamp  bolts  requir- 
ing no  drilling  of  holes.  Adds  weight  to  front  end  of  tractor  and  makes  steer- 
ing easier. 

that  up  to  a  certain  height  it  is  increasingly  difficult  to  throw  the 
tractor  over  backward.  With  wheel-type  tractors,  on  the  contrary, 
the  higher  up  the  front  of  the  tractor  is,  the  less  force  it  takes  to 
turn  it  on  over,  in  general. 

It  should  be  remembered  also  that  the  load  by  its  resistance  pulls 
backward  on  the  tractor  just  as  hard  as  the  tractor  pulls  forward. 
This  resisting  force  tends  to  revolve  the  tractor  over  backward  on 
the  line  through  the  places  where  the  drives  grip  the  ground.1  This 
tendency  to  revolve  is  doubled  by  doubling  the  height  of  the  drawbar 
above  the  ground.  Hence  with  a  low  drawbar  it  would  be  small; 
but  it  would  not  be  zero  unless  the  drawbar  were  down  at  the  level 


iThis  effect  is,  of  course,  present  in  tractors  that  drive  on  all  four  wheels. 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

of  the  grousers  or  lugs  in  the  surface  of  the  soil.  This  would,  of 
course,  not  be  practical.  A  horse  when  pulling  hard  has  the  same 
tendency  to  tip  up  in  front.  Often  a  horse  is  enabled  to  pull  his 
load  out  of  a  hard  place  by  the  assistance  afforded  him  when  his 
driver  hangs  onto  the  hame  or  rides  on  his  shoulders. 

Careful  tests  have  shown  that  on  level  ground  it  is  practically 
impossible  to  turn  over  backward  any  tractor  now  on  the  market 
unless  we  have  all  or  nearly  all  of  the  following  conditions  present : 


Fig.  4. — Loading  front  of  tractor  by  casting  masses  of  concrete  in  the  front 
wheels.  Use  rich  concrete  mixture,  reinforcing  with  heavy  iron  wire  fastened 
to  the  spokes. 


(1)  Too  little  of  the  tractor's  weight  carried  in  front. 

(2)  No  governor,  or  a  governor  not  working  properly. 

(3)  A  very  high  maximum  gear  reduction  ratio   (i.  e.,  when 
in  low  gear)  between  engine  and  drive  wheels. 

(4)  A  high  drawbar. 

(5)  Engine  racing  to   its  limit. 

(6)  Clutch  thrown  in  quickly,  or  grabbing. 

The  manufacturer  is  responsible  for  the  first  four  points;  the 
operator  for  the  last  two.  With  the  last  two  present  it  may  be  en- 
tirely possible  to  make  certain  tractors  rise  in  front  and  turn  on 
over  backward  when  they  are  on  level  ground  and  even  when  not 
attached  to  any  load  other  than  the  weight  of  the  tractor  itself. 


Circular  267]    tendency  of  tractors  TO  risk  jx   fkoxt  5 

A  tractor  having  the  first  four  characteristics  may  be  operated 
safely  (1)  by  care  in  handling  the  clutch  and  throttle  and  {2)  by 
lightening  the  load  before  the  drivers  have  dug  themselves  in.  It 
may  be  made  still  safer  (3)  by  adding  weight  in  front,  figs.  3,  4, 
and  4A.  This  is  best  done  by  adding  a  flange  fin  to  each  front  wheel, 
riveting,  bolting,  or  clamping  it  to  the  fin  already  on,  if  there  be  one. 
These  extra  fins  may  be  obtained  from  manufacturers,  from  most 
tractor  service  stations,  or  may  be  forged  from  %//x21/2//  iron.  The 
added  weight  of  the  flanges  and  their  deeper  penetration  will  also  make 


Fig.  4A. — Loading  front  of  tractor  by  bolting  into  the  'front  wheels  pieces 
of  cast  iron,  which  may  be  filled  with  concrete  if  desired. 


Patented 


Fig.  5. — Auxiliary  drawbar  for  Fonlson  tractors.  Furnishes  a  drawbar  free 
to  swing  between  horizontal  guides  and  helps  to  prevent  tipping  over  back- 
ward. 


6  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

steering  easier  and  more  certain.  Another  and  cheaper  way  of  adding 
weight  is  by  casting  masses  of  concrete  in  the  front  wheels,  as  shown 
in  figure  4.  A  rich  concrete  mixture  should  be  used,  reinforcing  with 
iron  wire  fastened  to  the  spokes.  The  concrete  must  be  kept  moist  at 
least  a  week  while  setting  or  it  will  speedily  go  to  pieces  from  the 
jarring. 

Cast  iron  pieces  are  obtainable  (fig.  4A)  which  may  be  bolted 
into  the  light  tractor  wheel  to  add  to  the  weight.  They  may  be 
filled  with  concrete  if  desired  and  would  effectively  prevent  the 
concrete  from  shaking  out.  The  castings  shown  in  the  figure  weigh 
sixty  pounds  to  a  wheel. 


Fig.    6. — Auxiliary    drawbar    for    Fordson    tractors.     A    swinging    drawbar 
also  makes  for  easier  turning  with   such   loads  as   disk   harrows,   drags,   pul- 


The  hitch  point  may  be  lowered  by  bolting  on  an  auxiliary  draw- 
bar, either  stationary  or  swinging.  The  markets  afford  several  kinds 
of  extra  drawbars  designed  for  use  with  certain  tractors  to  make 
them  easier  to  steer  and  to  assist  in  overcoming  sidedraft  effects. 
Some  of  these,  also,  while  not  especially  designed  for  that  purpose, 
render  the  tractor  safer  against  tipping  over  backward  when 
operated  by  careless  drivers. 

The  drawbars  shown  in  figures  5  and  6  are  in  part  designed  to 
serve  this  purpose.  A  drawbar  extending  out  some  distance  back 
of  the  rear  axle  may  prevent  tipping  over  backward  by  the  anchorage 
it  affords  when  attached  to  a  heavy  load,  for  example,  a  subsoiler. 
It  is  entirely  possible  to  cause  certain  four  wheel  tractors  to  run 
along  for  considerable  distances  with  the  front  wheels  up  a  foot  or 


CIRCULAR  267]      TENDENCY    OF    TRACTORS   TO    RISE    IN    FRONT  7 

more  in  the  air,  the  tractor  drawbar  and  frame  lining  up  with  the 
line  of  resistance  of  the  load.  It  would  of  course  be  impossible  to 
steer  the  tractor  under  such  circumstances.  Tt  should  also  be  pointed 
out  that  a  drawbar  extending  rearward,  if  pinned,  increases  side- 
draft  effects.2 


Fig.  7. — Extra  swinging  drawbar  designed  for  Samson  tractor. 

There  are  numerous  devices3  obtainable  which  are  designed  to 
' '  kill  the  engine "  or  to  release  the  clutch  of  a  tractor  when  the  front 
wheels  have  risen  a  certain  distance  above  the  level.     Some  of  these 


2See  Bulletin  No.  349  Calif.  Agr.  Exp.  Sta.  A  study  of  Sidedraft  and 
Tractor  Hitches. 

3Local  dealers  in  tractors  and  farm  machinery  are  in  general  able  to 
furnish  tractor  accessories  that  help  to  overcome  rising  in  front,  or  at  least 
they  can  supply  addresses  of  the  manufacturers.  Manufacturers'  addresses 
may  also  be  obtained  upon  application  to  the  Division  of  Agricultural  Engin- 
eering,   Branch    of   the    College    of   Agriculture,    Davis,    California. 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

devices  may  prove  of  value  under  certain  circumstances,  but  the 
remedy  for  the  trouble  is  rather  to  be  found  in  a  thorough  under- 
standing of  the  causes  and  a  careful  avoidance  of  them.  In  general 
it  may  be  said  that  loading  down  a  tractor  with  supposed  safety 
devices  makes  for  greater  complexity  and  hence  for  greater  chance 
for  accident.4 


■n 

■■p 

•           '■  '  V  ■■;. 

!■'■ 

i"\    %■ 

; 

-1,          ' 

j 

ii— i" " 

18-             ^ 

^^            "1 

In 

m 

* -~&f*\ 

~~t-;             ^  ^^   %j 

JNwB 

l^*^\ 

5^ 

•I, 

'*& 

|p-  - 

1 

\ 

^Jf>M 

.'?■, 

:■>■       :. 

Fig.  8. — Under  view  of  the  drawbar  of  figure  7,  showing  details  of  construction  and 

attachment. 


-^Discussions  of  this  subject  from  a  mathematical  standpoint  by  E.  B. 
Hewitt  and  D.  L.  Arnold  may  be  found  in  Transactions  of  the  Society  of 
Automotive  Engineers,  1919,  Part  1,  pp.  83-95;  and  by  Adams  in  Common  Sense 
Instructions  on  Gas  Tractor  Operation,  p.  209 


